1. Field of the Invention
The present invention relates to blind rivet fasteners and, in particular, to a blind rivet fastener in which the primary load is transferred through the insertion head thus limiting damage to the composite materials being joined and enabling the fastener to be utilized with panels of various thicknesses.
2. Description of the Prior Art
Three piece blind fasteners have been known and used for many years, the three pieces consisting of a nut threaded onto a bolt and an expandable sleeve located therebetween. Prior three piece blind fasteners were found to encounter significant problems when utilized with sheets of material which are relatively soft or material such as a graphite composite or when utilized in structures in which very thin sheets of material are utilized on the blind side. In particular, the force applied by the sleeve on the blind side surface would be sufficiently high to deform the blind side surface and weaken the sheet at that point. Blind rivet fastener assemblies have been developed to achieve a large bearing surface on the blind side so that such materials could be properly fastened with minimal damage to the blind side. However, disadvantages still are associated with their use. For example, U.S. Pat. No. 4,203,346 to Hall et al. discloses a fastener which produces a relatively small bearing surface on the blind side particularly when compared to the length of sleeve employed. In addition, the fastener has a tendency to delaminate a portion of the back surface. Moreover, the internal taper of the sleeve bore increases its cost to manufacture.
U.S. Pat. No. 4,457,652 to Pratt provides a blind fastener which seeks to overcome the disadvantages of the prior art as exemplified by the Hall et al. patent by providing a blind fastener which comprises a nut with an axial bore therethrough and with a conically shaped nose at one end and an enlarged head at the other end. Extending through the nut is a bolt having a stem and having an enlarged head adjacent to the nut nose. Between the nut nose and the bolt head is a sleeve on the bolt stem. The sleeve has a thick wall section adjacent to bolt head and a thin wall section adjacent to the nut nose formed by a recess in the section. A collar is provided on the bolt stem between the nut nose and the sleeve, the collar having a tensile strength less than the tensile strength of the sleeve. The sleeve is adapted upon the setting of the fastener to initially expand its thin wall section over the nut with the collar therebetween, then to have the free end of the thin wall section taper inwardly toward the nut. Thereafter the thin wall section buckles outwardly upon contact with the blind surface of the parts being fastened to form a large bearing surface.
Although the Pratt patent purports to provide an improved blind fastener assembly particularly adapted for use with composite materials, a significant amount of load is transferred to the composite bearing surface during the installation process, thus increasing the probability that damage will occur to the blind side surface.
U.S. Pat. Nos. 4,312,613 and 4,364,697 to Binns disclose blind fastener assemblies which have specific configurations such that controlled buckling is provided to increase the bearing area, thus reducing the possibility of failure of the composite material which forms the workpieces to be joined. However, as with the Pratt patent, a significant amount of load is still transferred to the composite bearing, or blind, surface.
U.S. Pat. No. 2,324,142 to Eklund discloses an early blind fastener assembly design which, although disclosing bulbing (buckling) to hold the parts riveted in place, is not concerned with parts fabricated from composite materials.
What is thus desired is to provide an improved blind fastener assembly for joining parts made of composite materials and which significantly reduces and controls the load transferred to the composite bearing (blind) surface during the installation process enabling the fastener to be utilized to secure together composite sheets, or panels, of various thicknesses.